I use "petgold"-supercal. It's Petco's version of Nutrical. My male, Jig, loves it. He's a little scrawny when you feel his body to the point I can feel his pelvic bones kind of protruding so I give him about a half teaspoon every time he turns his nose up to one of his feedings. He's a picky eater. I do not give it to my female, she's a hearty eater and is very solid.
The only time it has had any adverse reactions was when I first started giving it to him, he had a little diarrhea. When I talked to the vet, his only concern was possible diarrhea.
I just saw an advertisement in the Foster and Smith catalog for their version of nutri-cal called vitacal. The price was $2.99. I just may be giving that one a try because the petgold is about $8.00 a tube and I have to drive more than an hour to a Petco.
Dawn